In some power supply systems electric power is supplied from a power source to an electric or electronic device via a switchboard and a converter circuit. Some examples of the power source are a power network (an electrical grid), an electric generator (e.g. a diesel generator), batteries, solar cells and/or wind turbines. The electrical grid usually transfers electric power as an AC voltage (alternating current) whereas electric generators usually produce AC voltage (alternating current), and batteries and solar cells usually produce DC voltage (direct current) wherein a DC/AC inverter may be needed to convert the DC voltage to AC voltage before the power can be supplied to the switchboard. In these kinds of systems the electronic device may, however, operate with DC voltage, wherein a rectifier may be needed to convert the AC voltage to DC voltage before supplying the power to the electronic device. The rectifier may be a part of the electronic device or it may be a separate unit.
An example of the above mentioned power supply system has been used at many airports for supplying power to lamps located on and around runways and taxiways i.e. to the airfield lighting (AFL). These lamps or lights are used to illuminate location, layout, shape and purpose of runways and taxiways so that airline pilots can operate in all conditions, especially in dark, low light and low visibility conditions.
The amount of electrical power fed to an airfield lighting circuit may be regulated by adjusting the output current to the required level. This may be performed by a constant current regulator unit (CCR) so that the output current of the constant current regulator unit regulates also intensity of lamps at runways and taxiways. Several specified intensity levels are used as defined in related standards. Required airfield light intensity level for any given flight operation moment is defined by air traffic control at the airport. A required, regulated by standards taxiway lamp light intensity range is 2.8 A (1%)-6.6 A (100%). The lower light intensity level approx. 10% 0.66 A is practical minimum limit with some prior art LED lamps due to difficulties in implementing good power regulation performance for airfield lighting output current level at lower 10%-1% light intensity. This means that runway lights cannot be directly set to required minimum 1% level by such prior art LED lamps by only using constant current regulator unit output. Thus, a separate active power control function measuring airfield lighting circuit current and performing further power reduction is used inside each LED unit. FIG. 6 illustrates an example of illumination intensity as a function of input current for halogen lights (curve 601) and for led lights (curve 602). In this example, for halogen lamps the current level 4.1 A corresponds about 10% illumination level, the current level 3.4 A corresponds about 3% illumination.
Existing technical problem in some prior art at low LED light intensity levels: If current is controlled by PWM method, it may produce visible blinking and stroboscope effects which interfere airline pilots. Blinking is created by PWM pulse ratio, which has OFF time significantly longer than ON time. This blinking is more visible with lower AFL circuit frequency, e.g. supply network frequencies. Therefore, using PWM at low LED light intensity levels is today difficult as PWM adjustment resolution is getting practically too low.